1. Field of the Invention
The invention relates to the field of metal forming, and particularly to an improvement to the method and apparatus for forming structures, allowing a reduction in the size and costs of dies.
2. Description of Prior Art
It has been known for many years that certain metals, such as titanium, and other alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency toward necking. This capability is exhibited by only a limited number of metals and alloys, and within limited temperature and strain rate ranges. For example, some titanium alloys, such as Ti-6Al-4V, have been observed to exhibit superplastic characteristics.
Until the advent of viable superplastic forming techniques, taking advantage of this property to form complex configurations requiring large tensile elongations was extremely difficult, or in some instances, not possible. A significant breakthrough in superplastic forming was made by Hamilton, et al., disclosed in U.S. Pat. No. 3,934,441, Controlled Environment Superplastic Forming, incorporated into this specification herewith by reference. Simplified, the process involves placing a metal blank workpiece over a cavity in a die. The blank is heated to a temperature where it exhibits superplastic characteristics after which pressure is applied to the blank, causing it to stretch and form into the cavity. This varying pressure is reacted by the tooling and a force reactor system, typically in the form of a hydraulic press.
Another example of superplastic forming is disclosed in U.S. Pat. No. 3,340,101, Thermoforming of Metals, by Fields, Jr., et al.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a sufficient time so as to cause co-mingling of the atoms at the joint interface. Diffusion bonding is accomplished entirely in the solid state at or above one-half the base metal melting point. Actual times, temperatures and pressures will vary from metal to metal.
The combining of superplastic forming and diffusion bonding (SPF/DB) in the making of metallic sandwich structures has been successfully accomplished and is disclosed in U.S. Pat. No 3,927,817, Method of Making Metallic Sandwich Structure, by Hamilton, et al., and is herein also incorporated by reference.
Basically, the Hamilton, et al. method for making metallic sandwich structures involves fabricating the structures from a plurality of metal blank workpieces. One or more of the blanks are coated in selected areas not to be diffusion bonded. The blanks are positioned in a stacked relationship and placed in a die assembly wherein the stack is contained at its periphery forming a seal thereabout. The stack is diffusion bonded together in the uncoated areas by the application of controlled pressure, and at least one of the blanks is superplastically formed against one or more of the die surfaces forming the sandwich structure. The core configuration is determined by the location, size and shape of the joined areas. Another example of SPF/DB is disclosed in U.S. Pat. No. 4,087,037, Method of and Tools for Producing Superplastically Formed and Diffusion Bonded Structures by Schieve et al. and British Pat. No. 1,398,929, Joining and Forming Sheet Metal Members by L. E. A. Summers, et al.
All the processes involve containing one or more workpieces between a pair of dies with the workpieces creating a gap between the pair of dies. The dies are clamped about the preform, either by direct mechanical fastening to each other as disclosed in U.S. Pat. No. 3,340,101, Thermal Forming of Metals by Fields, Jr. et al., or by placing the dies and workpiece in a press between two platens as disclosed in U.S. Pat. No. 3,927,817, Method of Making Metallic Sandwich Structures, by Hamilton, et al. Regardless of which method is used, once the die and preform are clamped in place, they are effectively restrained from further movement. This creates a problem because the forming operations are carried out at high temperatures. For example, when superplastic forming titanium alloys, such as Ti-6Al-4V, the forming temperatures are around 1600.degree. F. Thus, both the pair of dies and preform will expand. This may create high loads in the dies, and in order to minimize these loads the dies are generally fabricated from a high temperature resistant stainless steel which is expensive and difficult to machine.
Because of the expansion of the dies during forming temperatures, and additionally, the fact that the pressures within the die required for forming typically vary throughout the forming cycle, it is difficult to insure that the structure being formed remains in tolerance, i.e., the structure may distort by virtue of inappropriate die pressure. For example, die pressure which is too high can cause flow forming of the constrained periphery of the part while die pressure which is too low, can cause the seal to be lost or slippage of the preform while forming. Thus, in the past, to be sure that the structure remained in tolerance, excessive clamping force was used, aggravating the problem.
Past forming apparatus have not addressed the problem of regulating a preselected force on the dies and maintining it as internal pressure varies during the forming cycle to insure that only the minimum force necessary to insure that the structure being formed will be within tolerance is applied to the die. For example, U.S. Pat. No. 3,742,537, Static Press by R. E. Merrill, discloses a press comprised of a support plate containing an inflatable hose-like member about its periphery. Coupled to the hose is a mold die adapted to mate with an upper plate. The mold die is brought into contact with the upper plate by inflating the ring. The pressure for forming is obtained by applying pressure between the lower plate and mold die with the ring acting as a seal. While the force against the upper plate can be varied by varying the gas pressure, Merrill does not disclose a method whereby the pressure can or should be regulated to a preselected value during the forming cycle. Furthermore, there is no apparatus for determining the pressure that should be applied during the forming cycle.
Another example can be found in U.S. Pat. No. 3,667,891, Molding Press by R. J. Geltin. Geltin discloses a press having lower and upper platens with a pair of dies mounted in between. A bolster plate is movably mounted on a support frame. Between the bolster plate and the upper platen is a plurality of parallel inflatable flat tubes. In operation, the bolster plate is brought down and locked in proximity of the upper platen and the flat tubes are inflated to apply pressure to the upper platen and subsequently to the die. Here again, while the pressure could be varied by changing the pressure inside the plurality of flat tubes, Geltin also does not disclose an apparatus to accomplish it. Another example of the use of a movable platen with an inflatable member adapted to apply pressure to the die can be found in U.S. Pat. No. 3,376,808, Fluid Operated Press by D. E. Beckett, et al.
It is, therefore, a primary object of this invention to provide a method and apparatus for forming parts with the application of a regulated preselected die clamping force.
It is another object of this invention to provide a method and apparatus for forming parts using smaller and less expensive dies.
It is a further object of this invention to provide a method and apparatus for more accurately producing parts by superplastic forming and superplastic forming/diffusion bonding.